U.S. Pat. No. 4,303,601, Ventilator Humidifier, to Grimm, et al. [hereinafter cited as Grimm], discloses a ventitilator, humidifier device for gases having an automatic liquid level sensing device. Coupled to the liquid level sensing device is an automatically operated valve which allows added liquid to flow into a ventilator, humidifier system thereby maintaining liquid at a desired optimum level. A removable cannister for containing the liquid has a closure defining a gas inlet and a gas outlet as part of the flow line to the breathing apparatus of the patient. A tubular wick inside the cannister soaks up the aqueous liquid.
In Grimm, liquid is maintained typically at a relatively low level within the cannister so that the circulating gas is humidified primarily from the tubular wick. More liquid may be added to the cannister, for example, from a solution bag of sterile water, through a tube into the cannister. The tube generally passes through a solenoid switch which pinches the tubing shut to stop flow and intermittently opens the tube when a low liquid level is sensed. A light probe is positioned in the cannister so that visible light, or typically light from the infrared portion of the spectrum, passes down the probe and is reflected from a conical-shaped end of the probe in the event that the liquid level is low. A sensor detects the reflected light and actuates the solenoid to open the switch. Liquid then flows into the cannister until reflected light is no longer detected.
U.S. Pat. No. 4,354,984, Liquid Level Controller for a Humidifier, to Richardson, et al. [hereinafter cited as Richardson], discloses a tube clamp assembly for an oxygen gas humidifier used in respiratory therapy. The clamp assembly is part of a control system for controlling the flow of replacement liquid to the humidifier cannister. A light probe and sensor are a part of the liquid level control system.
Typically, the light probe, or light pipe, used to detect liquid level, is made from a hydrophobic, thermoplastic material. The liquid level sensing probe has an end which may be defined by a conical surface if desired, or any other surface which provides angled surfaces positioned to permit light passing into the highly transparent probe and toward the end surface to be reflected back up the probe in a returning beam of light after striking at reflection points.
When the end of the light probe of the Grimm patent is in contact with liquid, the refractive conditions at the end surface are changed because of the relatively slight difference in the index of refraction of liquid with respect to the light probe so that the light beam is not reflected. Instead, the light passes through the surface, continuing its downward path so that there is no significant reflective light beam. Accordingly, the presence or absence of contacting liquid at the end surface can be indicated by the presence or absence of a reflected light beam.
A light sensor, in the Grimm patent, communicating with the other end of the light probe detects the reflected light. When a reflected light beam is detected, generally indicating a low liquid level, the light sensor actuates a solenoid to open a valve permitting liquid flow through tubing to the cannister. The liquid level rises until the end surface of the probe is covered. At this point, the reflected beam terminates, thus deactivating the light sensor and causing the solenoid to deactivate. This, in turn, closes the valve, shutting off liquid flow.
At times, submerged air or other gas bubbles adhering to the end surface of the light probe may cause the light beam to be reflected back through the light probe and detected by the light sensor even when the liquid level is adequate. The light sensor would, in turn, actuate the solenoid to open the valve, permitting liquid to flow into the cannister. The addition of liquid, however, when submerged air or other gas bubbles adhere to the light pipe, would not cause deactivation of the solenoid. Liquid would continue to flow into the cannister until a local disturbance dislodged the adhered gas bubbles, until the liquid source was depleted, or until a specified time period elapsed.
Gas bubble formation or nucleation generally is caused by air or another gas fixing to a site and adhering to microscopic fissures, cavities, scratches and other surface irregularities. Generally, gas bubble formation occurs when the surface tension of the aqueous liquid is greater than the surface tension of the light probe.
Sometimes, beads and droplets of liquid adhere to unsubmerged portions of the light probe. These forms of liquid will refract internally reflected light away from the light probe. A dramatic loss of totally internally reflected light results, and there is an undesirable loss of the light signal necessary for the detection of liquid level. A light probe having an exposed end and having liquid droplets adhering thereto may refract enough internally reflected light away from the light pipe so that liquid is not maintained at an optimum level through controlled liquid replacement. Liquid droplet formation on unsubmerged portions of the light probe occurs when the surface tension of the condensate is greater than the surface tension of the light probe.
It would be desirable to alter the surface tension characteristics of an optical surface which is operational with aqueous liquids and which optical operation is impaired by the effects of the indices of refraction among the liquid, gases, and the optical surface.
It would be expedient to eliminate or greatly reduce the adherence of submerged gas bubbles to instruments operational with aqueous liquids whereby the operation is impaired by adherence of gas bubbles.
It would be advantageous to alter the surface tension characteristics of the surface of a light probe used in a humidifier container which would make the light probe surface tension greater than the aqueous liquid with which it is used in order that gas bubbles in the aqueous do not adhere to the light probe thereby producing erroneous indications of liquid level.
It would be advantageous to alter the interiors of containers enclosing the basic elements of a ventilator, humidifier device in order that the effect would be to render surfaces of enclosed elements more hydrophilic in order that microscopic beads and droplets of liquid do not adhere to unsubmerged portions of enclosed devices and in order that gas bubbles in the aqueous liquid do not adhere to submerged portions of the enclosed devices.
It would be expedient to eliminate or greatly reduce the adherence of beads and droplets of liquid to optical instruments whereby operation is impaired by adherence of liquid droplets.
It would be advantageous to alter the surface tension characteristics of an optical surface which is operational in an environment conducive to liquid droplet formation which operation is impaired by their effects on the refractive indices among the liquid, gases, and the optical surface because of the differences in the indices of refraction between the aqueous liquid and the optical surface.
It would be desirable to alter the surface tension characteristics of the surface of a light probe used in a humidifier container which would make the surface more hydrophilic in order that beads and droplets of liquid do not adhere to unsubmerged portions of the light probe but form a thin layer of liquid parallel to the surface of the probe so that internally reflected light is not refracted away from the light probe.
It also would be advantageous to provide a substantially non-toxic agent which may be added to an aqueous liquid whereby the formation of gas bubbles or liquid droplets is inhibited on instrumentation operational with the aqueous liquids. Operation of the instrument would not, thereby, be impaired by the adherence of submerged gas bubbles or droplets of liquid on unsubmerged portions of the instrument.